What is 3D Laser scanning?

3-D laser scanning is a relatively new, but already revolutionary and very successful surveying technique. The two techniques mostly used in terrestrial surveying are the time-based and the phase-based techniques. Whether a time-based or phase-based laser scanner is used in a survey, both yield a digital data set, which is essentially a dense “point cloud”, where each point is represented by a coordinate in 3-D space (X, Y and Z, relative to the scanner’s position). With this data, the 3-D shape of any object or geometry of a scene can be quickly determined. The most important advantage of the laser scanning method is that a very high point density can be achieved, depending of the laser scanner type and distance to the object. In any case, the shape of the surveyed object or scene can in principle be measured in three dimensions at a very high level of detail and accuracy.

Time-based laser scanners

Today, the most popular measurement system for laser scanners is based on the time-of-flight principle, which will be referred to as time-based scanner. This technique allows measurements of distances up to several hundreds of metres or more, under ideal circumstances. The accuracy and precision of the measured distances is in the order of 1 centimetre or better, depending on the distance to the target. The time-based scanners -or sometimes called "ranging" scanners - have a laser diode that sends a pulsed laser beam to the scanned object. The pulsed laser beam moves through a rapidly changing zenith and azimuth angle in response to the motion of a mirror inside the instrument. The pulse is diffusely reflected by the surface of the scene or object and part of the light returns to the receiver. The time that light needs to travel from the laser diode to the object surface and back is precisely measured. Knowing the speed of light, the distance from the scanner to the object and the azimuth and zenith angle of the beam, the position of each point where the beam is reflected can be calculated to three-dimensional Cartesian space. In addition to the range and angular measurements, which are translated on-the-fly into the X, Y, Z coordinates, the amplitude of the returned signal is also recorded which is often referred to as intensity.

Phase-based laser scanners

The other type of laser scanner, namely phase or modulated-based systems, work in a different manner. Instead of having the light source pulsing on and off they run it constantly. The light source is modulated with a sine wave, causing the amount of light that the laser emits to vary accordingly. In similar fashion to the time-of-flight method, the signal is transmitted from the laser and reflected from the object. To determine the length measured, the phase differences between the transmitted signal and the reflected signal are compared. The range is restricted to a maximum of seventy to one hundred metres. Accuracy and precision of the measured distances within some millimetres is possible and is in this respect better than the range-finding or time-based laser scanner. The speed of measurement is also much higher, up to 100 times faster than time-of flight laser scanners. This allows the phase-based type of laser scanner to be used on mobile platforms for rapid surveying, for example mounted on a railway carriage inside a tunnel.

Rapid face mapping and documentation

Rapid face mapping for geological and geotechnical applications

Documentation of slope current stata

Geometric analyses of slopes and earth bodies

Volume calculations

Monitoring of movement / slope instability/rock fall detection

Underground (tunnel) survey

Survey of underground space geometry

Detection of discontinuities, loose blocks in wall and roof

Measurement of orientations in exposed rock faces

Surface reconstruction approach

Point cloud segmentation approach

Deriving joint set spacing

Measurement of discontinuity roughness

Small-scale roughness

Large-scale roughness

Open pit mining

3D modeling

Real-time volume monitoring

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

 

1     Introduction

2     Principle of 3D terrestrial laserscanning

2.1     Generation of a 3D point cloud

2.1.1       Time-based approach

2.1.2       Phase-based approach

2.2     Laser scanner types

2.2.1       Time-based types

2.2.2       Phase-based types

2.3     Examples of laser scanners

2.3.1       Time-based laser scanners

2.3.2       Phase-based laser scanners

3     Accuracy and precision issues

3.1     Accuracy

3.1.1       Laser beam divergence

3.1.2       Scanning resolution

3.2     Precision

3.2.1       Distance error

3.2.2       Angular error

3.3     Survey distance

3.3.1       Reflective properties of rock surfaces

3.3.2       Laser energy, laser beam divergence

3.4     Specifications of laser scanners

3.4.1       Riegl

3.4.2       Leica

3.4.3       Optech

4     Operational issues

4.1     Registration

4.1.1       Absolute registration

4.1.2       Relative registration

4.2     Field-of-View

4.3     Merging of multiple scans

4.4     Occlusion

4.5     Combination with photographic (colour) information

4.5.1       Orthorectification of colour imagery

4.5.2       Colouring of the point cloud data

4.6     Noise in the data

4.6.1       Influence of vegetation

4.6.2       Influence of dynamic disturbances

4.7     Meteorological influences

4.7.1       Haze

4.7.2       Dust

4.7.3       Temperature

4.7.4       Sunlight

5     Applications

5.1     Rapid face mapping and documentation

5.1.1       Rapid face mapping for geological and geotechnical applications

5.1.2       Documentation of slope current stata

5.2     Geometric analyses of slopes and earth bodies

5.2.1       Volume calculations

5.2.2       Monitoring of movement – slope instability/rock fall detection

5.3     Underground (tunnel) survey

5.3.1       Survey of underground space geometry

5.3.2       Detection of discontinuities, loose blocks in wall and roof

5.4     Measurement of discontinuity orientations in exposed rock faces

5.4.1       Surface reconstruction approach

5.4.2       Point cloud segmentation approach

5.5     Open pit mining

5.5.1       3D modeling

5.5.2       Real-time volume monitoring

6     Current developments

6.1     Hardware

6.1.1       Accuracy and precision developments

6.1.2       Integration with other survey instrumentation

6.2     Software

6.2.1       Generic point cloud handling software

6.2.2       Dedicated geotechnical and geoscience software

6.2.3       Integrating photogrammetric information

7     Research directions

8     Conclusions

Dear Colleague, We invite you to join the 3D Laser Scanning Commission (Nr. 19) of International Association for Engineering Geology and the Environment (IAEG). Researchers, students and professionals interested in the application of 3D terrestrial laserscanning in the Geosciences are welcome to join.

Individuals interested in joining the commission works may attend one of the Commission meetings. Please contact the Secretary (Siefko Slob) if you would like to be included in the mailing list of the Commission. In that way you will always be informed about our latest activities.

The main objective of this Commission is to provide a framework where codes of practice and working methods of this new technology can be worked out. The target group are (earth science and civil engineering) professionals working within various areas on geoscience-related projects where precise documentation and measurements are needed from the site investigation stage to the construction stage.

The laser scanning technique will provide accurate and precise geometry as an input for 3D numerical models and CAD drawings. 3D laser scanning also allows creating very realistic 3D visualisation and animation of objects and scenes, which improves the communication within a project and to the outside world.

Best regards,

Kennert Röshoff (Chairman)

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